When a signal travels along a path that has an impedance discontinuity (or “mismatch”), the signal is partly reflected. The reflected signal interferes with the original signal and this can result in a loss of signal integrity and an incorrect signal level being detected by a receiver. To mitigate the onset of signal reflection, it is beneficial to place circuitry with the equivalent amount of impedance at the point of discontinuity. This is referred to as “termination”. For example, resistors can be placed on computer motherboards to terminate high speed buses.
Although termination resistors reduce reflections at an extremity of the signal path, they are unable to prevent reflections resulting from stub lines that connect to other semiconductor chips at various points along the path. This situation can arise, for example, when multiple memory modules are connected along a memory bus. A signal propagating from a memory controller along the memory bus encounters an impedance discontinuity at each stub line leading to a particular memory module. The signal that propagates along the stub line leading to the particular memory module will be reflected back onto the memory bus, thereby introducing unwanted noise into the signal.
Accordingly, it is useful to provide each semiconductor chip with its own termination circuitry. Providing this termination circuitry on the same semiconductor chip that includes a bus transmitter an/or receiver is known as on-die termination (ODT). On-die termination can reduce the number of resistor elements and complex wirings on the motherboard. Thus, in addition to improved signal integrity, which allows components to be operated at higher frequencies, on-die termination enables a simpler and more cost effective system design.
However, conventional on-die termination techniques have tended to be power hungry and/or inflexible.